Effect of high-temperature and rapid pyrolysis condition of rice husk on the evolution of char structure

Abstract

The conversion level of carbon in the process of biomass char utilization determines the overall efficiency of subsequent utilization, while the conversion rate of biomass char depends to a certain extent on the physical and chemical structure of biomass. In order to explore the evolution of char structure during high-temperature rapid pyrolysis of rice husk, a high-temperature rapid pyrolysis system of rice husk was designed and built. Through the accurate control of the residence time and temperature of rice husk pyrolysis, the reaction process can be accurately controlled. The physical and chemical structure of rice husk char was characterized by scanning electron microscope, Raman spectroscopy, specific surface area analysis and Fourier transform infrared spectroscopy. The yield of rice husk char, the type and development of pores, the changes of chemical functional groups and carbon structure under different pyrolysis temperatures and residence times were studied. The results were analyzed to obtain the trends of yield, pore types and development, chemical functional groups, and carbon structure of rice husk char at different pyrolysis temperatures and residence times. When the pyrolysis temperature is 1 300 ℃ and residence time is 13 s, the maximum specific surface area of rice husk char is 141.17 m2/g, the pore size is mainly distributed between 2−10 nm, and the surface structure is obviously broken. With the increase of pyrolysis temperature and residence time, the chemical structure of rice husk char becomes more and more deoxidized. The long residence time drives the highly ordered char structure of rice husk char and the concentration of aromatic rings with six or more dense benzene rings in rice husk char increases. High temperature significantly damaged the C—O bond and —OH bond in rice husk char, resulting in the breaking of the branch chain and bridge bond, release of oxygen-containing functional groups and graphitization of the carbon skeleton. The surface carbon elements were mainly dominated by C—C/C—H, and the content of carbon increases from 63.86% at 900 ℃ to 68.27% at 1300 ℃ when the residence time is 5 s. However, the contents of C—H, C—O, C=O and COO— decreased to different degrees. The surface oxygen elements were mainly dominated by COO and COOH. As the pyrolysis temperature increased, the number of benzene rings in rice husk char gradually decreased, the average number of aromatic rings increased, the number of surface methylene groups decreased, and the carbon activity decreased. Summarizing the structural evolution law of rice husk char will provide a reference for optimizing pyrolysis conditions, improving pyrolysis efficiency and subsequent combustion and gasification utilization of rice husk char.

Keywords

• rice husk
• high-temperature pyrolysis
• char structure
• pore structure
• functional group structure